RE: whats the difference between .308 and a 7.62mm X 39mm, or are they the same?

European cartridge designation makes more sense than the US designations, which don't mean anything. European designation is bullet diameter and length, expressed 7.62x39 (nominal .30 caliber bullet, case length is 39 mm). 7.62 Nato, AKA .308, is 7.62x51. the 7.62x54 noted above is actually 7.62x54R...the "R" designates a rimmed cartridge.
Just a quick look at the two cartridges in question will point out the different case sizes.
7.62x39, and 7.62x51.

RE: whats the difference between .308 and a 7.62mm X 39mm, or are they the same?

the 7.62x39 does fire a .311 bullet.....

The 7.62x39 Soviet (Model 43)
By Chuck Hawks

This attenuated .303 cartridge (bullet diameter is .311", not .308" like true 7.62mm rifles) has had one of the most infamous careers in the history of firearms. It was designed during the Great Patriotic War (World War II to those of us in the Free World) for use on the Eastern Front against Hitler's Wehrmacht by Stalin's Communist hoards.